The present invention relates to storing computer information.
It is known to store computer information on magnetic tape driven by streaming tape drives. Information is stored on the tape by using a write head to change the magnetic condition of the tape as the tape moves across the write head. This information is stored in units known as blocks, where between each block is a blank area, i.e., a gap, where no information is stored. Information is read from the tape by using a read head to sense the changes of flux produced by the local magnetic condition of the tape as the tape moves across the read head. A tape which moves linearly across the read and write heads without stopping in the gaps between blocks is a "streaming" tape.
Because information is stored serially on the tape in a uniform pattern, information must be written to and read from the tape at a rate corresponding to the speed at which the tape drive operates. Streaming efficiency and throughput depend on continuous tape motion. Therefore, if the rate of host computer transfer is below that required to maintain streaming operation of the drive, efficiency drops due to the necessity of repositioning the tape, a phenomenon particular to streaming tape drives. This condition is defined as an underrun condition. Additionally, if the rate of host computer transfer is above that required to maintain streaming operation, efficiency drops due to overflowing a data buffer. This condition is defined as an overflow condition.
This problem has been addressed by equipping tape drives with two speeds. If the processor can not transfer data at the higher drive speed and repositioning is frequent, the drive resorts to a lower speed to prevent repetitive underrun (this lower speed is usually 1/3 or less the higher rate). In this type of system, the higher speed of the drive is usually slower than what the drive is ultimately capable of in order to minimize using the lower speed. This may, however, result in frequent overrun. The higher speed is chosen to stay below the "typical" processor transfer rate; thus, favoring overrun as opposed to underrun, because underrun requires repositioning and/or speed change.